The present invention relates to a method and an arrangement for positioning the pistons of pressure-fluid cylinder devices.
Cylinder devices used for manouvering purposes and operated by fluid under pressure--e.g. compressed-air piston-cylinder devices, are often used to produce linear movements. The pistons of such devices are almost exclusively set to work between two fixed stop positions, normally the two end positions of piston travel. When the piston is intended to stop at a position intermediate of its two end positions, it is necessary to provide a brake arrangement or some other kind of mechanical stop. This is because the pressure fluid--the air--is compressible and cannot therefore hold the piston in an exact, fixed position when the piston is subjected to an external force or load which may vary intermittently or with time.
The presence of a brake arrangement or a mechanical stop, however, causes the piston to start and stop in pronounced jerks, due to the fact that the fluid pressure applied to the piston is sufficient to prevent the piston from "sinking back" at the start, and because the piston cannot be stopped in a correct position, simply by lowering the pressure.
The problem is well known and, consequently, when there is a desire or a need to position the piston of a piston-cylinder device in definite, selected positions, there is most often used a cylinder device which works with an incompressible fluid--hydraulic fluid. With such devices, the supply of pressure fluid--oil--is blocked with the aid of valves such as to lock the piston in the position desired. Hydraulic systems are encumbered with certain drawbacks, however, including high installation costs and high operating and maintenance costs. Furthermore, there is always a risk that the hydraulic oil will leak from the device and therewith create problems.
With the intention of eliminating the drawbacks associated with such piston-cylinder devices, Swedish Patent Application 8504344-6 (which corresponds to U.S. Pat. No. 4,771,674) proposes a method for positioning the piston of a piston-cylinder device with the aid of a pressure detector which detects the load acting on the piston and which is operative to send a control signal to a pressure regulator which, in turn, controls the pressure in the cylinder, so as to constantly to balance the force or forces acting on the piston. This enables the piston to be held in an exact, determined position, irrespective of whether the piston is subjected to load. Although this piston-positioning arrangement functions satisfactorily in practice, it is unnecessarily complicated and expensive. The arrangement is also prone to malfunction.